The present invention relates to a tantalum thin film usable as interconnections and electrodes in liquid crystal displays and electronic devices, and more particularly to a low resistive tantalum thin film structure and a method for forming the same.
In recent years, tantalum Ta has been widely used for metal-insulator-metal devices acting as switching devices in active matrix liquid crystal displays, and for interconnections for driving thin film transistors or electrodes since tantalum has the following advantages. First, tantalum is anodizable and thus may be made by anodization into tantalum oxide (Ta.sub.2 O.sub.5) which has an extremely excellent insulating property. Second, tantalum is sufficiently resistant to chemicals and corrosion. Third, tantalum is highly adjustable to manufacturing processes.
If tantalum is deposited by normal sputtering on an insulation substrate to form a tantalum film, then a resistivity of the tantalum film is relatively high, for example, in the range of 170 .mu..OMEGA.cm to 200 .mu..OMEGA.cm. If an impurity gas such as nitrogen is mixed in a discharge gas, then a resistivity of tantalum film is normally about 60 .mu..OMEGA.cm.
Particularly when it is required to enlarge a screen of the liquid crystal display and improve a high definition, one of the most important issues is how to reduce the material for interconnections used for driving the liquid crystal display. In those circumstances, the requirement for further and possible reduction in resistivity of tantalum has been on the increase.
In prior art, it has been proposed to provide a lamination structure of a tantalum nitride thin film and a tantalum thin film formed on the tantalum nitride thin film. Such techniques are disclosed, for example, in the Japanese laid-open patent applications Nos. 3-293329, 5-48097 and 5-289091. The thickness and nitrogen content of the tantalum nitride thin film are important factors for obtaining a low resistivity tantalum thin film. In the Japanese Laid-open Patent Application No. 5-289091, it is disclosed that an electrode is provided on an insulating substrate, wherein the electrode comprises a lamination structure of a tantalum nitride film and a tantalum film of body centered cubic crystal structure overlying the tantalum nitride film. It is proper that the tantalum nitride film has a thickness in the range of from 10 angstroms to 1000 angstroms and the tantalum nitride film has a nitrogen concentration in the range of 7 atm to 13 atm % or 33 atm % or more so that a resistivity of the tantalum film overlying the tantalum nitride film is 25 .mu..OMEGA.cm.
In the Japanese laid-open Patent Application No. 3-293329, the following is disclosed. The thickness of the tantalum nitride may be approximately 300 angstroms or less. Particularly if the conditions for sputtering are optimized, the thickness of the tantalum nitride film may be 50 angstroms or more. If a nitrogen concentration of the tantalum nitride film is 40% or more, then the resistivity of the tantalum thin film overlying the tantalum nitride film is 30 .mu..OMEGA.cm. Since the nitrogen concentration depends on the sputtering apparatus and the sputtering conditions, the nitrogen concentration should not necessarily be limited to the above range.
In the Japanese laid-open Patent Application No. 5-48097, it is disclosed that if the thickness of the tantalum nitride film is in the range of several tens angstroms to a few hundred angstroms, then the resistivity of the tantalum thin film overlying the tantalum nitride film is approximately 20 .mu..OMEGA.cm.
The Japanese laid-open Patent Application No. 5,48097 describes the reason why the resistivity of the tantalum thin film overlying the tantalum nitride film is reduced as follows. If the tantalum thin film is epitaxially grown on the tantalum nitride film having the body centered cubic crystal structure, the tantalum thin film also has .alpha.-phase, for example, the body centered cubic crystal structure and further is free of any impurity.
In the Japanese Laid-open Patent Applications Nos. 3-52264 and 6-194677, it is disclosed that the tantalum nitride film and the tantalum thin film are laminated in order to improve adhesion to the substrate.
The Japanese Laid-open Patent Application No. 3-52264 describes as follows. In order to increase adhesion force of a tantalum electrode to an insulating substrate, the thickness of the tantalum nitride film is in the range of 100 angstroms to 2000 angstroms and the nitrogen concentration thereof is in the range of 0.5% to 10%.
On the other hand, the Japanese Laid-open Patent Application No. 6-194677 describes as follows. The nitrogen concentration of the tantalum nitride film is 10 atm % or more. The sputter powers for growing the tantalum nitride film and the tantalum thin film are limited to 4.0 W/cm.sup.2 per a unit target area to suppress any raise of substrate temperature.
The Japanese Laid-open Patent Application Nos. 5-289091 and 6-194677 describe the methods for forming the tantalum nitride film, for example, sputtering by using a tantalum nitride alloyed target and reactive sputtering by using a mixed gas of argon and nitrogen. The above other applications also describe that the tantalum thin film overlying the tantalum nitride film may also be grown by sputtering, following to the growth of the tantalum nitride film by sputtering.
The Japanese Laid-open Patent Application No. 3-293329 further describes that the tantalum nitride film may be formed by subjecting the tantalum film to plasma nitration or thermal nitration in place of the sputtering.
Moreover, in the Japanese Laid-open Patent Application No. 59-55016, conventional methods for forming a refractory metal nitride film are disclosed as follows. A refractory metal is sputtered using a high frequency power onto a semiconductor substrate to form a metal nitride film which is useful as a barrier metal thereon. If the high frequency discharge is used for sputtering, a larger reduction in resistivity of the refractory metal nitride film can be obtained rather than when the dc discharge sputtering is used. The reduction in pressure of the sputtering gas can reduce the resistivity of the refractory metal nitride film. Preferable range of the sputtering gas pressure is in the range of 5 mTorr to 10 mTorr.
It was, however, confirmed by the inventors of the present application that if the tantalum nitride film is grown by the reactive sputtering, then it is difficult to obtain a sufficient reduction in resistivity of the tantalum thin film overlying the tantalum nitride film. It has been found that in order to obtain a further reduction in resistivity of the tantalum thin film it is insufficient to control the thickness and the nitrogen concentration of the tantalum nitride film in accordance with the conventional manner as described above.
The method for growing the tantalum nitride thin film by direct sputtering using the tantalum nitride alloyed target has problems in that it is difficult to make the target and to generate many particles. Further, nitrogen components in the target are volatilized during formation of the film, or the scattering effect of nitrogen elements is enlarged in case of the tantalum nitride alloy target. For those reasons, the nitrogen concentration during the growth of the film may vary depending upon the sputtering conditions and condition in use of the target whereby the resistivity of the tantalum film overlying the tantalum nitride film is unstable.
When the method for forming the tantalum nitride film by subjecting the tantalum thin film to a plasma nitration or by thermal nitration is used, then an increased number of process steps are required. When the thermal nitration is carried out, then the substrate receives thermal damage. Tantalum has the .beta.-phase, for example, cubic lattice structure which provides a high resistivity. Even if such tantalum is subjected to the plasma nitration or the thermal nitration, the undesirable cubic lattice structure remains unchanged to still have a high resistivity. Namely, it is difficult to obtain a sufficient reduction in resistivity of the tantalum thin film overlying the tantalum nitride film.
In the above circumstances, it had been required, until the preset invention was made by the inventors, to provide a novel lamination structure of a tantalum nitride film and a tantalum thin film having a reduced resistivity, in addition to provide low resistive tantalum interconnections and low resistive tantalum electrodes.